NEW YORK -- Terrorists carried out the most destructive attack on the United States in history yesterday, a horrifying rain of four hijacked airliners that toppled both towers of the World Trade Center in New York and destroyed a section of the Pentagon.

Although the death toll won't be known for days, thousands were believed dead, including 266 aboard the planes, hundreds of firefighters and police missing in the rubble in New York and scores of people in the 1,300-foot-plus-high towers, where about 50,000 worked.

At the Pentagon, a source said, "the services believe they have fewer than 500 unaccounted for," including military, civilian and contract personnel.

Buildings in Manhattan's financial district were still burning late last night, as was a part of the Pentagon.

The fiery assault on prominent symbols of the nation's financial and martial strength jolted the nation. Airports, schools, government buildings and virtually every place a large crowd might gather were closed, and comparisons were made to the bombing of Pearl Harbor in 1941.

This time the enemy is unknown and the response uncertain, although President Bush signaled last night in a nationwide address that he hasn't ruled out a broad retaliation, saying, "We will make no distinction between the terrorists who committed these acts, and those who harbor them."

As Bush vowed to "hunt down and punish those responsible for these cowardly acts," others were grappling with how to strike back against zealous, shadowy foes.

Their weapons of choice were Boeing passenger jets -- two 767s and two 757s, each taking off from an East Coast airport within a few minutes of 8 a.m., each loaded with enough flammable fuel to make it to California and each with the same array of flight controls, suggesting that the hijackers had been trained to pilot the planes.

The apparent sophistication of the operation immediately focused investigative attention on Osama bin Laden, the exiled Saudi who has been linked to the 1993 bombing of the World Trade Center and the 1998 bombings of U.S. embassies in Kenya and Tanzania.

Sen. Orrin G. Hatch, a Utah Republican, said U.S. intelligence officials had overheard bin Laden associates discussing an attack. "We happened to know just today that we have information that indicates representatives that are affiliated with Osama bin Laden were actually saying over the airwaves, private airwaves that they had hit two targets," Hatch said.

A U.S. official familiar with the information was more cautious. "There are indications that people with links to bin Laden and the Al Qaeda organization may have been responsible, but it is still premature," he said.

Warning received

Abdel-Bari Atwan, editor of the Al-Quds al-Arabi newspaper in London, said he received a warning from Islamic fundamentalists close to bin Laden but did not take the threat seriously. "They said it would be a huge and unprecedented attack, but they did not specify," Atwan said.

In Afghanistan, where bin Laden has been given asylum, the hard-line Taliban rulers condemned the attacks yesterday while attempting to deflect blame from bin Laden, saying he didn't have the necessary resources.

By late yesterday, the only identifiable resources employed in the attacks were careful planning and a willingness to die.

The terrorists struck first aboard American Airlines Flight 11, a Boeing 767 that left Boston for Los Angeles at 7:59 a.m. with 92 crew members and passengers aboard. One or more persons then hijacked the plane and apparently took the controls, changing course for New York.

Barbara Olson, a television commentator and wife of U.S. Solicitor General Ted Olson, a passenger on one of the later hijacked flights, described the hijackers' methods in a phone call to her husband before the plane crashed, according to several reports.

She said the hijackers were armed with knives and cardboard cutters, and that they herded passengers, pilots and crew members to the back of the plane.

Attorney General John Ashcroft told members of Congress that three to five terrorists were aboard each hijacked plane. Some reports said the hijackers forced passengers with cellular phones to call relatives, announcing that they were about to die.

At 8:45 a.m., with an estimated 20,000 people at work inside the two 110-story towers of the World Trade Center, the American Airlines jet out of Boston plowed into the upper third of the north tower, sending flames and black smoke billowing into the blue sky.

Lawyer Coleman Nutter, 52, was working on the 58th floor, about 20 stories below the impact.

"I heard it, and I also felt it," Nutter said. "I looked out my window and saw paper, debris and glass falling down. I went to the stairs and started to walk down, but it was packed." It took him a harrowing half-hour to make the trip to the ground floor.

By then, a second flight bound from Boston's Logan Airport to Los Angeles -- United Airlines Flight 175, a Boeing 767 with 65 aboard -- had also been hijacked and had also turned south toward Manhattan.

Grace Schalkwyk, 45, senior vice president at Standard & Poors, saw the jetliner approaching from her 34th-floor office a few blocks from the trade center.

"The plane came toward the building, turned on its side and went right into the [second tower]," she said. "Flames came out right away."

By then, much of the country was watching with her, on televisions at their homes, schools and offices, witnessing images destined to take their place alongside vivid memories of assassinations and moon landings as signature events of their generations.

Among those who weren't watching were people frantically trying to make their way to safety. Mary Gensheimer was heading down the stairs from her office on the 78th floor of the south tower, having just seen the explosion after the crash next door when the second plane hit her building.

'The building swayed'

"The building swayed," she said. "I honestly thought the building was going to fall over. Then the panic set in. ... We ran down, people were throwing their shoes off. ... Some people were screaming to move over. I held two women who I didn't know who were sobbing. I walked them down."

She reached the street to find panicky crowds, gasping and shrieking, heads turned toward the drama unfolding far above.

"I could count 30 or 40 people just dropping out of windows," said filmmaker Paul Lemos, 35. "It was horrifying. ... I kept praying for each soul as they were falling down. I just couldn't believe it was happening. I could see a lot of people standing on the ledges and holding hands. Then one at a time they would jump."

Those closest to the scene were trying to move to safer ground, dodging the falling debris.

"Glass shards were falling, and all kinds of stuff," said Ephraim Juskowicz, 27, a securities analyst for First Albany Corp. who grew up in Pikesville. "I put my work bag up over my head. Everyone was running away. People were screaming; some were crying."

The situation worsened, not only there but in the skies. The next target was Washington, and two more jets had been hijacked -- American Airlines Flight 77, a Boeing 757 bound from Washington Dulles International Airport to Los Angeles with 64 aboard, had made it as far as the Midwest before being turned back toward Washington. United Airlines Flight 93, a Boeing 757 bound from Newark, N.J., to San Francisco with 45 aboard, had been hijacked and turned east over Ohio.

At 9:45 a.m., the Dulles flight zeroed in on the Pentagon from the south, clipping light poles in the vast parking lot before plowing into the outer wall of the building, where 20,000 employees work. The plane exploded.

Five minutes later, the south World Trade Center tower collapsed, folding in on itself in a cascade of glass and rising dust, blowing ashes worthy of a volcano across Lower Manhattan.

Hyman Brown, a University of Colorado civil engineering professor and the construction manager for the World Trade Center, speculated that flames fueled by thousands of gallons of aviation fuel melted steel supports.

"This building would have stood had a plane or a force caused by a plane smashed into it," he said. "But steel melts, and 24,000 gallons of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire."

Air control towers across the country began calling all flights from the sky, grounding those that hadn't taken off. Ten minutes after the first World Trade Center tower collapsed, the last of the hijacked planes, the United flight out of Newark, headed for the ground.

Passenger calls

In Pennsylvania, an emergency dispatcher received a cell phone call at 9:58 a.m. from a man who said he was a passenger locked in a bathroom aboard United Flight 93, said dispatch supervisor Glenn Cramer in Westmoreland County. The man repeatedly told officials that the call was not a hoax. "We are being hijacked! We are being hijacked!" he said.

Cramer said the man aboard the plane told dispatchers that it "was going down. He heard some sort of explosion and saw white smoke coming from the plane, and we lost contact with him." The plane crashed in a field at the edge of a forest, about 80 miles southeast of Pittsburgh.

A half-hour later, the second of the trade towers collapsed in New York.

By then, some blocks of the financial district were ankle-deep in ash and strewn papers, smoke blocking the sun. Possibly buried in the rubble of the final collapse were entire units of the New York City Fire Department. In late afternoon, a 47-story building near the trade towers also collapsed.

The walking wounded were shuttled to Liberty State Park in New Jersey, and Manhattan came to a standstill, closing the entry tunnels and bridges to traffic, and shutting down the subways and trains.

The Boston Herald, quoting a source it did not identify, reported last night that authorities had seized a car at the airport that contained Arabic-language flight training manuals.

The source said five Arab men had been identified as suspects, including a trained pilot. At least two of those men flew to Logan on yesterday from Portland, Maine, according to the Herald. The luggage of one of the men who flew to the airport didn't make his scheduled connection.

The Boston Globe reported the luggage contained a copy of the Koran, an instructional video on flying commercial airliners and a fuel consumption calculator. The FBI refused to comment on the reports.

Following the plane crashes, caution took hold.

Evacuations were ordered at the United Nations in New York and at the Sears Tower in Chicago. Los Angeles mobilized its anti-terrorism division. Walt Disney World in Orlando, Fla., was evacuated, and Hoover Dam on the Arizona-Nevada line was closed to visitors. Security was tightened along the 800-mile trans-Alaska oil pipeline.

As the initial shock subsided, the questions began.

"This is comparable to Pearl Harbor," said former Secretary of State Henry A. Kissinger, "and we must have the same response, and the people who did it must have the same end as the people who attacked Pearl Harbor."

Republican Sen. Charles E. Grassley of Iowa predicted that the military response "will be major. ... We've got to send the signal that we are not going to tolerate this attack on our nation."

U.S. leaders weren't the only ones who compared the attack to warfare.

"This is a declaration of war against the entire civilized world," said German Chancellor Gerhard Schroeder, pledging "full solidarity" with the United States.

Not all corners of the world shared such sentiments.

In Nablus, a Palestinian town on the West Bank, many residents celebrated and tossed candy, seeing the attack as a rare blow struck against the nation supporting Israel's opposition to their cause. Palestinian Authority President Yasser Arafat condemned the attack.

In some quarters of the United States, life will begin returning to normal today, with the reopening of some schools and most government buildings. About 150 members of the Senate and Congress attempted to set the tone by gathering on the steps of the Capitol shortly after nightfall to sing "God Bless America."

Airlines will remain grounded at least through noon today, White House officials announced, and federal and local officials in New York and Washington will continue searching for bodies in the rubble.

Record toll likely

Given the numbers of people at the scenes of yesterday's disasters, the death toll seems destined to set U.S. records for carnage.

The death toll aboard the crashed planes alone -- 266 -- would surpass that of the Oklahoma City bombing April 19, 1995, which claimed 168 lives in what was then the deadliest act of terrorism on U.S. soil.

Members of a Maryland family were among the passengers aboard Flight 77. They were Leslie A. Whittington, an associate professor of public policy at Georgetown University, her husband, Charles Falkenberg, and their two young daughters, residents of University Park. They were en route to Australia, said school spokeswoman Julie Green-Bataille. Whittington was "a tremendously respected and loved member of our community," she said.

The Japanese attack on Pearl Harbor killed 2,403 and wounded 1,178. The worst one-day total of casualties suffered in any U.S. military action was approximately 23,000 Americans killed and wounded on both sides in the Battle of Antietam during the Civil War.

Sun staff writers Jean Marbella, Michael Stroh, Mark Bomster, Gady A. Epstein and wire services contributed to this article.

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